Paraffin had been used many years as an embedding medium in the preparation of tissue specimens for sectioning in a microtome to produce specimen sections for histological studies. Such embedding processes generally include the well known steps of specimen fixation, dehydration, clearing, paraffin infiltration or impregnation, blocking or embedding in a block of paraffin, slicing the block and specimen into thin sections, mounting the sections on slides, removing the paraffin and solvents employed for this purpose, and staining the sections. These are then readly for microscopic analysis. The primary purpose of the embedding medium is to permit the specimens to be sectioned and mounted in the natural state.
Some of the problems of using plain or ordinary paraffin wax, as purchased from the manufacturers of paraffin, as a specimen embedding material, have been that a relatively long period of time is required to adequately infiltrate the specimen and the inability to obtain a section as thin as desired and one that is flat and wrinkle-free. Paraffin tends to collapse and crumble, and especially when a relatively hard or brittle object is struck during sectioning. The compressibility of plain paraffin generally increases the chance of deforming the specimen section, and the thermal and mechanical shock properties are not as good as generally desired.
In attempts to improve paraffin as an histological embedding material and overcome some of the above mentioned problems, additives, such as various resins, beeswax and polymers, have been added. However, paraffin waxes differ greatly in thermal and mechanical characteristics since they are derived from crude oil taken from the earth at different locations throughout the world. Paraffin varies especially in content of linear and cyclic hydrocarbons which causes the large variations in mechanical and thermal properties among paraffin waxes suppled by different companies. Because of the differences in the paraffins, additives have resulted in improved paraffin embedding compositions but only when added to certain available paraffins. A serious problem has been the poor market availability of paraffin waxes which are suitable for use in the manufacture of emedding media. For example, some paraffin waxes are reltively soft and generally cannot be sliced sufficiently thin without tending to crack and crumble even when such additives were used.
In general, good paraffin based embedding materials now being marketed are capable of producing good specimen slices having thicknesses between about 2 and 5 microns. Where slices having a thickness of 1 micron are required, methacrylate and acrylate ester embedding materials have sometimes been used. However, the necessary polymerization reaction and the need of a special microtome having a glass knife blade to avoid dulling are prolems associated with the use of the latter materials. Paraffin based embedding media generally has not been entirely satisfactory for sections less than 2 microns in thickness and is relatively poor for 1 micron sections.